isthmus: visit(NamedWrite) ignores output_mode, so a write declaring MODIFIED_RECORDS converts to a ROWCOUNT TableModify

[nielspardon]

visit(NamedWrite) never reads getOutputMode() on the INSERT and DELETE paths -- only handleCreateTableAs checks it, for CTAS -- and returns a LogicalTableModify whose row type is a single ROWCOUNT column:

substrait-java/isthmus/src/main/java/io/substrait/isthmus/SubstraitRelNodeConverter.java

Lines 1100 to 1107 in 934a60e

	return LogicalTableModify.create(
	targetTable,
	requireCatalogReader(relOptSchema, write.getNames()),
	input,
	operation,
	null,
	null,
	false);

The plan says otherwise. AbstractWriteRel.deriveRecordType() returns the input's record type, and the spec documents OUTPUT_MODE_MODIFIED_RECORDS as returning the records the operator inserted, deleted or updated, further manipulable upstream. So a write declaring MODIFIED_RECORDS converts to a relation that reports one column where the plan declares the input's, and a parent reading it resolves its references against columns that are not there.

isthmus produces those plans itself -- the export direction stamps the mode on every INSERT and DELETE:

substrait-java/isthmus/src/main/java/io/substrait/isthmus/SubstraitRelVisitor.java

Line 798 in 934a60e

.outputMode(AbstractWriteRel.OutputMode.MODIFIED_RECORDS)

#1189 refuses an emit mapping on a write because the mode is dropped, and its message says so, but the drop itself is still silent. Either refuse MODIFIED_RECORDS the way the mapping is now refused, or model it.

Found while reviewing #1189.